linux-kernel - Re: [PATCH v5 3/3] KVM: LAPIC: Apply change to TDCR right away to the timer

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Date:   Fri, 6 Oct 2017 22:03:53 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v5 3/3] KVM: LAPIC: Apply change to TDCR right away to the timer

2017-10-06 21:03 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 2017-10-06 07:14+0800, Wanpeng Li:
>> 2017-10-06 2:14 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
>> > 2017-10-05 07:35-0700, Wanpeng Li:
>> >> From: Wanpeng Li <wanpeng.li@...mail.com>
>> >> +             remaining = ktime_sub(apic->lapic_timer.target_expiration, now);
>> >> +             if (ktime_to_ns(remaining) < 0)
>> >> +                     remaining = 0;
>> >> +             delta = mod_64(ktime_to_ns(remaining), apic->lapic_timer.period);
>> >> +
>> >> +             if (!delta)
>> >> +                     return false;
>> >> +
>> >> +             apic->lapic_timer.period = (u64)kvm_lapic_get_reg(apic, APIC_TMICT)
>> >> +                     * APIC_BUS_CYCLE_NS * apic->divide_count;
>> >
>> > I'd prefer to apply the rate limiting (done earlier in this function) to
>> > the period.  This version allows the guest to configure 128 times more
>> > frequent interrupts in the host.
>> > (And thinking about it, the version of [2/3] I proposed has similar
>> >  problem when switching from one-shot to periodic, only there it is
>> >  unpredictably limited by the speed of KVM.)
>>
>> We didn't stop and restart the timer, why the rate will influence us for [2/3]?
>
> It is because of the rate limiting -- the guest could setup a one-shot
> timer with a short expiration and switch to periodic

Yeah, in addition, I think configure 128 means more slower interrupts
instead of faster.

Regards,
Wanpeng Li

>
> It is mostly theoretical as the expiration would have to be long enough
> so that the timer doesn't fire before KVM emulates the next instruction
> that switches the timer to periodic mode, but shorter than rate limit.
>
> I see you handled that in v6, thanks!